Autism is a neurodevelopmental disorder that results in abnormalities of social and language development and is associated with rigid and repetitive behaviors. Although there is strong evidence of heritability, the involved genes have not been identified. The prevalence of autism spectrum disorders may be as common as 1 in 166. The average concordance rate in monozygotic twins is 70% suggesting that environmental factors play a role in the disease. Subgroups of autistic children seem unusually sensitive to infections, immunizations and dietary factors, but none of these factors has been causally identified with the disease. Nevertheless, autoimmunity has been considered to play a role on the basis of indirect evidence. There is no evidence-based efficacious treatment for autism (Folstein and Rosen-Sheidley, 2001). There is a subgroup of children with autism that appear to develop typically for a period of time, and then lose skills, or regress. A recent study by Vargas and co-workers at Johns Hopkins has demonstrated that the regressive subtype of autism is associated with chronic brain neuroinflammation as exemplified by activation of microglia and astroglia and the abnormal production of inflammatory cytokines and growth factors assayed in both tissue samples (brain banks) and CSF. The authors remarked that these responses were similar to those seen in some neurodegenerative disorders such as amyotrophic lateral sclerosis, and that ?chronic microglia activation appears to be responsible for a sustained neuroinflammatory response that facilitates the production of multiple neurotoxic mediators.? Chronic neuroglial activation could be the result of an abnormal persistence of a fetal development pattern. In this scenario neuroglial activation could play a role in initiating and in maintaining the pathology. Alternatively, neuroglial activation may only be a secondary response to the initiating causal factor(s) and not a direct effector of injury. Since neuroglial activation requires the nuclear translocation of the pro-inflammatory transcription factor NF-kappaB, and since inhibitors of NF-kB with good CNS penetrance are available, the role of neuroinflammation in initiating and sustaining the autistic condition can be probed. The antibiotic minocycline is a powerful inhibitor of microglial activation, apparently through blockade of NF-kB nuclear translocation (Si et al., 2004). Minocycline is neuroprotective in mouse models of amyotrophic lateral sclerosis (ALS) (Van Den Bosch et al. 2002) and Huntington?s disease (Chen, M. et al., 2000) and has been recently shown to stabilize the course of Huntington?s disease in humans over a 2-year period (Bonelli, R.M. et al., 2004). To evaluate the possibility of benefit in autistic children, we propose to conduct an open-label trial of the anti-inflammatory antibiotic minocycline, an agent that reduces inflammation by blocking the nuclear translocation of the proinflammatory transcription factor NF-kB. Minocycline is Food and Drug Administration (FDA)-approved for treatment of a variety of infections and has been widely used for the treatment of adolescent acne. Minocycline is currently in phase III trials for the treatment of Huntington?s disease and amyotrophic lateral sclerosis. This proposal is for an initial 6-month, single-arm, off label, open-label study (with a 3 month extension phase offered to responders) that will evaluate dose safety and efficacy of minocycline in 10 children, ages 3 to 12 years, with a primary diagnosis of autism and a history of developmental regression. The subjects will be evaluated by a diagnostic/behavioral assessment, and the extent of neuroinflammation judged by CSF cytokine/chemokine profiles before and after the 6-month treatment. Subjects will also be given 0.6 mg/kg vitamin B6 twice a day as a prophylactic for possible minocycline induced nausea and vomiting. If the results of this feasibility study are encouraging, we expect to conduct a double-blind, placebo-controlled trial of minocycline therapy.